In a multiple antenna communication system, user equipment utilizes pilot frequency to perform channel estimation, and utilizes values of channel estimation to calculate CSI such as CQI (Channel Quality Information), PMI (Pre-coding Matrix Information), RI (Rank Information) and the like. After such CSI is fed back to a base station by the user equipment, it can be used by the base station for scheduling of the user equipment, and for selecting of a suitable coding modulation manner, a pre-coding transmission manner and the like for the scheduled user equipment.
The LTE (Long Term Evolution) system and the LTE-A (Long Term Evolution-Advanced) system adopt a same resource allocation structure. As shown in FIG. 1, a system bandwidth may be divided into n BPs (Bandwidth Parts), each of which in turn contains k subbands. When the CSI is fed back by the user equipment, the CSI corresponding to the subband whose CQI is optimal in each BP, together with the index of the subband, shall be fed back to the base station.
In the LTE system, there are four modes of CSI feedback in a control channel:
Mode 1-0; Mode 1-1; Mode 2-0; and Mode 2-1.
For the fourth modes, each contains four transmission types as follows.
Type 1: subband CQI, i.e., the channel quality information of the subband. The CQI prescribed in the LTE standard is an index of a coding modulation manner. The base station can determine the coding modulation manner according to the CQI fed back by the user equipment.
Type 2: wideband CQI+wideband PMI, i.e., the channel quality information of the wideband and the pre-coding matrix information of the wideband. The base station selects a pre-coding matrix according to the value of wideband PMI.
Type 3: RI, i.e., the rank information. The base station obtains the number of data flows according to this information.
Type 4: wideband CQI, i.e., the channel quality information of the wideband.
The transmission types corresponding to each of the feedback modes are as shown in Table 1.
TABLE 1PUCCHPUCCH modetypeFeedbackMode 1-1Mode 2-1Mode 1-0Mode 2-0numbercontentsMode status(bits/BP)(bits/BP)(bits/BP)(bits/BP)1SubbandRI = 1NA4 + LNA4 + LCQIRI > 1NA7 + LNA4 + L2Wideband2 transmitting antennas, RI = 166NANACQI/PMI4 transmitting antennas, RI = 188NANA2 transmitting antennas, RI > 188NANA4 transmitting antennas, RI > 111 11 NANA3RI2-layer spatial multiplexing11114-layer spatial multiplexing22224WidebandRI = 1 or RI > 1NANA44CQIIn Table 1, NA indicates that such information is not transmitted, and L is the index of the subband.
As shown in Table 1, taking the feedback mode 2-1 as an example, the information to be fed back is:
1) Subband CQI, the number of bits of which is 4+L or 7+L;
2) Wideband CQI/PMI, the number of bits of which varies depending on the number of the transmitting antennas, for example, is 11 if there are 4 antennas; and
3) RI, the number of bits of which is 1 or 2.
The other feedback modes are similar to those described above.
FIG. 2 gives a feedback instance for the feedback mode 2-1. The parameters are defined as follows.
1) J: the number of BPs corresponding to the system bandwidth one by one.
2) K: the number of times that the subband with the optimal CQI in the BP occurs during two wideband feedbacks.
3) Np: the time interval of subband CSI feedback.
4) H*Np=(J*K+1)*Np: the feedback period of the wideband CSI.
Firstly, the base station configures the parameters K and Np by a high layer such as the MAC (Media Access Control) sub-layer or the RRC (Radio Resource Control) sub-layer, and transfers such parameters to the user equipment. The configuration may be based on information on the present moving speed of the user equipment or the like. For example, if the user equipment is nearly at rest, the feedback period of the feedback subband CSI may be made long, that is, the value of Np and/or the value of K may be increased. The user equipment determines the feedback period of the wideband CSI according to the parameters J, K, and Np.
As shown in FIG. 2, the feedback period of wideband CQI+wideband PMI (Type 2) in the feedback mode 2-1 is H*Np=18 ms, and the feedback time interval of the corresponding subband with the optimal CQI in each BP is 2 ms, that is, Np=2. There are two feedback chances for each BP between two feedbacks of Type 2, that is, K=2.
In the discussion of the present LTE-A system, it has decided that the mode of the CSI feedback in the PUCCH (Physical Uplink Control CHannel) is natural extension of the CSI feedback mode prescribed in the LTE system. However, the LTE-A system can not adopt directly the feedback mode of the LTE system for the following three reasons.
1. In the LTE-A system, there is a pilot frequency CSI-RS (CSI Reference Symbol) dedicated for estimating of CSI. The CSI-RS generally has two transmission periods: 10 ms and 5 ms. In contrast, in the LTE system, the pilot frequency CRS for estimating of CSI is transmitted in each subframe, and is also used for channel estimation at the receiving end, for demodulating of data. That is, the user equipment in the LTE system calculates and obtains the CSI to be fed back according to the CRS of various subframes at a plurality of CSI feedback positions, while the user equipment in the LTE-A system can only calculate and obtain the CSI at a plurality of feedback positions according to the CSI-RS in a certain subframe.
2. It has been decided in the LTE-A system that the subband PMI is fed back in the control channel, while the LTE system does not support that such information is fed back in the control channel. That is, additional subband PMI information shall be fed back in the LTE-A system compared with the LTE system.
3. The LTE-A system adopts double codebook structure, i.e., pre-coding matrix W=W1*W2. W1 is selected from codebook 1 and targets wideband/long-term channel properties. W2 is selected from codebook 2 and targets narrowband/short-term channel properties. Both W1 and W2 shall be fed back. In contrast, the LTE system adopts single codebook structure.
Therefore, it is desirable to provide a method and apparatus for CSI feedback in the LTE-A system which enable the uplink control signaling feedback structure of the LTE-A system to substantially reuse the feedback structure of the LTE system, thus addressing the feedback issue of the control signaling in the LTE-A system while resulting in few alterations of system software and hardware.